The present invention relates to the field of semiconductor substrate processing and, more particularly, to the monitoring of material thickness and etch and deposition rates during plasma etch and deposition processes of semiconductor substrates.
The manufacture of an integrated circuit device requires the formation of various layers (both conductive, semiconductive, and non-conductive) above a base substrate to form necessary components and interconnects. During the manufacturing process, removal of a certain layer or portions of layers must be achieved in order to form the various components and interconnects. This is commonly accomplished by means of an etching process. Etching techniques in use include wet, or chemical etching, and dry, or plasma etching. The latter technique is typically dependent upon the generation of reactive species from process gases that are impinged on the surface of the material to be etched. A chemical reaction takes place between the material and these species and the gaseous reaction product is then removed from the surface.
With reference to FIG. 1, creating plasma for use in manufacturing or fabrication processes typically begins by introducing various process gases into a plasma chamber 10 of a plasma reactor, generally designated 12. These gases enter the chamber 10 through an inlet 13 and exit through an outlet 15. A workpiece 14, such as an integrated circuit wafer is disposed in the chamber 10 held upon a wafer holder 16. The reactor 12 also includes a plasma density production mechanism 18 (e.g. an inductive coil). A plasma inducing signal, supplied by a plasma inducing power supply 20 is applied to the plasma density production mechanism 18, the plasma inducing signal preferably being an RF signal. A top portion 22, constructed of a material transmissive to RF radiation such as ceramic or quartz, is incorporated into the upper surface of the chamber 10. The top portion 22, allows for efficient transmission of RF radiation from the coil 18 to the interior of the chamber 10. This RF radiation in turn excites the gas molecules within the chamber generating a plasma 24. The generated plasma 24 is useful in etching layers from a wafer or for depositing layers upon a wafer as is well known in the art.
An important consideration in all etch and deposition processes is the monitoring of process parameters such as etch and deposition rate, film thickness and determining a time, referred to as the endpoint, at which to end the process. Common methods for monitoring plasma etch and deposition processes include spectroscopy and interferometry. Spectroscopic methods include monitoring the chemical species in the plasma chamber and detecting a change in the concentration of an emitting species in the plasma when one film layer is cleared during an etching process and the underlying film is exposed. This method is not useful however in several etch processes where an underlying film is not exposed. For example, in a gate etch process, a layer of polycrystalline silicon or amorphous silicon lies above a thin oxide layer. The polysilicon layer must be etched away leaving the thin oxide layer without causing any pitting or punch through to the oxide layer. In order to achieve this, the etch chemistry must be changed at a point before the polysilicon layer is cleared. Spectroscopy is also not useful in shallow trench isolation and recess etch processes.
Interferometric methods are disclosed in U.S. Pat. No. 5,450,205 to Sawin et al. and include laser interferometry and optical emission interferometry. In laser interferometry, an incident laser beam strikes an interface between a wafer and a chamber environment such as a plasma of the plasma chamber. A reflected beam is directed through a bandpass filter to a photodiode where an interferometry signal is recorded as a function of time. The bandpass filter prevents plasma emission from entering the photodiode while allowing the reflected laser beam to strike the photodiode.
In optical emission interferometry, the light generated by the plasma is used as the light source for interferometry. Light is collected from the plasma chamber with a lens and passed through a bandpass filter and into a photodiode. The bandpass filter defines the wavelength of light being used as the interferometric signal and blocks light at unwanted wavelengths to prevent the plasma background from reaching the photodiode. In both laser interferometry and optical emission interferometry, the etching rate and film thickness is easily calculated by detecting the time between adjacent maxima or adjacent minima in the interferometric signal.
The use of broadband light sources in interferometric methods is also well known in the art. U.S. Pat. No. 5,291,269 to Ledger discloses an apparatus for measuring the thickness of a thin film layer including an extended light source that forms a diffuse polychromatic light beam. The beam illuminates an entire surface of a wafer and is reflected off the wafer and passed through filters to form a monochromatic light beam that is projected onto a detector array. The monochromatic light beam displays an interference fringe pattern image on the detector array. This pattern is processed to create a map of measured reflectance data that is compared to reference reflectance data to generate a map of the thin film layer thickness over a full aperture of the wafer.
To undertake interferometric measurements through a plasma, it is necessary to remove the contribution of the plasma emission from the interferometer signal and thereby reduce the effect of this contribution upon the algorithms used to model the thin film structures on the wafer. Fluctuations in the plasma emission can also confound models used to determine the etch rate of films on the wafer. The use of laser interferometry greatly reduces sensitivity to plasma emission but limits measurement to a single wavelength. Optical emission interferometry techniques depend on the plasma emission itself and are therefore sensitive to fluctuations in the emission and the range of wavelengths available for analysis varies with the process chemistry. Methods using extended broadband light sources provide a range of wavelengths useful for analysis but generally suffer from problems of low signal to noise ratio and low intensity interferometric signals.
It would therefore be desirable to provide a method and apparatus for monitoring a plasma etch or deposition process that reduces the sensitivity of the detector to plasma emission but that allows for measurements over a broad range of wavelengths, and in particular, measurement in the ultraviolet region of the spectrum. Materials used in integrated circuit fabrication are generally more reflective in the ultraviolet range and the use of shorter wavelengths allows for greater resolution of the interferometric signal providing for increased accuracy in film thickness measurement.
Prior art ultraviolet light sources are typically extended sources and coupling light efficiently from these sources is optically difficult. Additionally, these sources tend to be monochromatic sources. Finally these sources typically have relatively low intensity thereby making the interferometric signal harder to detect above the plasma emission background.
It would therefore be desirable to provide a method and apparatus for monitoring a plasma etch or deposition process that provides a non-extended light source for generating light that is efficiently coupled into an optical system.
It would further be desirable to provide a method and apparatus for monitoring a plasma etch or deposition process that provides an interferometric signal having a broad spectral range, high intensity and a high signal to noise ratio.
Finally, it would be desirable to provide an apparatus for monitoring a plasma etch or deposition process including a light source having a longer lifetime than the extended broadband light sources of the prior art.